Abstract

Soil organic matter (SOM) biotic degradation can be coupled with the abiotic oxidation by Fenton reaction. Recalcitrant SOM degradation is catalyzed by phenol oxidases and peroxidases, while Fenton occurs in presence of iron. These two coupled mechanisms can explain the high rate of decomposition in temperate rain forest soils of southern Chile. Most forest occurs in Allophanic soils in the Andean range and kaolinitic-illitic soils derived from metamorphic uplift in Coastal Cordillera. Both soils (mean annual precipitation >5000 mm) have a quick redox fluctuations over scales of hours to weeks. In the present study we hypothesized that anoxi-abiotic process involving Fenton accounts for significant proportion of recalcitrant SOM oxidation in temperate rainforests soils. The objective was to study the C release by Fenton and biotic (enzymes activities) mechanisms in a microcosm experiment. Top mineral Ah horizons were sampled (5-15 cm) and incubated at 20 ºC in dark under anoxic conditions. Fenton was evaluated by adding different peroxide:Fe2+ ratios 5:1-10:1-20:1 at different pH 3.0-3.5-4.0. The release of CO2, H2O2 consumption and iron oxidation were measured. Hydroxyl radical (•OH) was estimated using a fluorescence probe (confocal microscope). Phenol oxidase and peroxidase enzymes activities were also measured. About 33% of total CO2 was released by purely abiotic C mineralization in both soils. Kaolinitic-illitic soil mineralized released three times more CO2 than allophanic soil in abiotic way, probably by the large presence of Fe. Peroxide:Fe2+ ratio 10:1 induced the highest C mineralization,> 43 % over the control at pH 3.5 in sterilized soil. Fenton enhanced the presence of •OH. Biotic mineralization through phenol oxidative activity increased linearly with Fe2+ concentration in the soil and the opposite being true for the peroxidase activity. Iron abiotic oxidation coupled to biotic organic matter decomposition contributes to explain the rapid turnover time of soil C across temperate rainforests soils ecosystems.